LF: Measuring field strength

[Dick Rollema <dickrollema@casema.nl>]

Again with some errors removed....

To All from PA0SE

Mal, G3KEV wrote:

If you have a receiver that covers 500 kHz, has an S-meter and can be run off batteries you could use that with the single turn loop.

But the S-meter must be calibrated first using a signal generator.

73, Dick, PA0SE

Hi Dick

This is the method that I use, the tricky part is getting the loop antenna calibrated accurately, more complicated than you think, for absolute accuracy.

 

The loop must not be loaded. Most SLM's have a possibility for a
high impedance input.  That may be a problem with a receiver used as
a SLM. On the other end the impedance of the loop is so low on 500 kHz,
in my own one turn loop 2.7 ohms,  that even when loaded with 50
ohms the output of the loop hardly differs from the open circuit voltage.
Keep the coax between loop and SLM or receiver as short as possible and
convenient.

You can't trust the S-meter of a receiver. So that has to be
calibrated.  The loop itself does not have to be calibrated. There
is a simple equation that gives the relation between field strength and
voltage induced in the loop. I tried to include the equation into an
earlier message but that was rejected. The equation comes out of Terman
and includes Greek letters. I will rewrite it in a way that hopefully is
acceptable to the censor.

e = (2 * 3.1 * E * N * A  * cos phi) / L  in which:

e = voltage induced in loop in volt.
E = field strength in V/m
N = number of turns in loop
A = area of the loop in square metres
phi is angle with respect to plane of loop.
L = wavelength in metres.

When measuring the loop must be turned for maximum signal; that makes cos
phi = 1.

I prefer to use one turn, so N = 1.
Using more turns increases the voltage induced. But the capacitance
between the turns could corrupt the result. Also the reactance of the
loop increases, raising its impedance and  making it more difficult
to avoid loading the loop.
Usually the voltage induced in one turn is high enough to be read on a
SLM.
There is no need to screen the loop because of its low
impedance.

I made  my own loop as a square with sides of 1 metre. That makes A
= 1.  Afterwards I found that I could only just get the loop into
the car.
So better do some measurements on the doors of your car before deciding
on the size of the loop.  

Measuring field strength is definitely not a precision business. There
are so many disturbing factors that it is of no use to aim for highest
accuracy. Hopefully the people at your Ofcom realise that as
well.

The best way is to take many readings on different locations, in
different directions and different distances, as Jim, M0MBU, has done on
137 kHz and use the average value as the result. .
Also you have to keep out of the near field of the antenna. A distance of
2 wavelengths from the transmitter usually is enough.

Finally an anecdote.
When I started measuring FS on 137 kHz I looked for places around my home
at some 4 kilometres distance or more where the field would not be
disturbed by overhead lines, underground cables, large metal fences etc.
A road with street lighting  was obviously unsuitable because there
was certainly an underground cable.  I finally choose a nice parking
lot near the border af a lake. Walking around with my portable instrument
showed that the reading did not change so the field was reasonably
undisturbed. I then made measurements with the one turn loop and the W
& G SPM-12 SLM, run off the car battery.
Sometime later I read that the place was a meeting point for homosexuals.
I then realised there were indeed always some cars with a man inside.
What must have been their thoughts about my mysterious doings?

Happy measuring!

73, Dick, PA0SE